fancy api caching action filter (for etags and max age)

EnableApiCachingActionFilter.cs

    /// <summary>
    /// Enables caching by setting cache-control to private (instead of no-cache).
    /// By default also enables etags, and sets cache-control max-age to 60 seconds.
    /// </summary>
    public class EnableApiCaching : ActionFilterAttribute
    {
        private readonly TimeSpan _maxAge;
        private readonly bool _enableEntityTagCaching;

        /// <summary>
        /// Enables caching by setting cache-control to private (instead of no-cache).
        /// By default also enables etags, and sets cache-control max-age to 60 seconds.
        /// </summary>
        /// <param name="enableEntityTagCaching">A boolean indicating whether or not to enable entity tag based caching.</param>
        /// <param name="maxAgeInSeconds">
        /// The maximum age in seconds that the cached response is valid.
        /// NOTE: this will be overridden by a request's x-requested-max-age header if provided.
        /// </param>
        public EnableApiCaching(bool enableEntityTagCaching = true, int maxAgeInSeconds = 60)
        {
            _enableEntityTagCaching = enableEntityTagCaching;
            _maxAge = new TimeSpan(0, 0, maxAgeInSeconds);
        }

        public override void OnActionExecuted(HttpActionExecutedContext context)
        {
            if (context.Request.Method.Method.ToUpperInvariant() != "GET" || context.Response == null) return;

            //we need public or private (not no-cache) for etags to work. 
            //We also tell it not to bother the server with a real request for maxAgeInSeconds.

            var ageToUse = _maxAge;

            if (context.Request.Headers.Contains("X-Requested-Max-Cache-Age"))
            {
                int requestedMaxCacheAge;

                if (int.TryParse(context.Request.Headers.GetValues("X-Requested-Max-Cache-Age").FirstOrDefault(), out requestedMaxCacheAge))
                {
                    ageToUse = new TimeSpan(0, 0, requestedMaxCacheAge);
                }
            }

            context.Response.Headers.CacheControl = new CacheControlHeaderValue { Private = true, MaxAge = ageToUse };

            //while competent HTTP caches such as the ones used in chrome and firefox will update cache entries when receiving 304 (Not Modified)
            //internet explorer/System.Net.Cache/WinInet won't (even though it spits out logs like it actually is).
            //Until this is resolved, HttpClient has have significantly worse performance after the initially cached response expires
            //compared to just using the cache control headers, as the cache is then never refreshed.
            //this wasted a ton of my time trying to figure out.
            //see http://stackoverflow.com/q/20925934/957829
            //if you're confident your api won't be consumed by the http client, you can probably safely use http client.

            //we use this fancy header to indicate not to send back etags, since httpclient can't handle these properly, but must other clients can.
            if (context.Request.Headers.Contains("X-Requested-No-ETag")) return;

            if (!_enableEntityTagCaching) return;
            if (context.Response.Content == null) return;

            var byteArrayTask = context.Response.Content.ReadAsByteArrayAsync();
            byteArrayTask.ConfigureAwait(false);
            byteArrayTask.Wait();

            var etag = BitConverter.ToString(MD5.Create().ComputeHash(byteArrayTask.Result)).Replace("-", "");

            context.Response.Headers.ETag = new EntityTagHeaderValue("\"" + etag + "\"");

            if (context.Request.Headers.IfNoneMatch.Any() &&
                context.Request.Headers.IfNoneMatch.First().Equals(context.Response.Headers.ETag))
            {
                context.Response.StatusCode = HttpStatusCode.NotModified;
                context.Response.Content = null;
            }
        }
    }